The present invention relates to a capacitor which is employed in a MMIC (Monolithic Microwave Integrated Circuit), etc. in circuit portions, etc. of a radio device such as a portable telephone, a mobile phone, etc., for example.
As shown in figures on pages 176, 177 of xe2x80x9cFoundation of Microwave Circuit and its Applicationxe2x80x9d (published by Sogo Denshi Publishing Co., Ltd., Feb. 1, 1992), for example, as for the conventional capacitor constructed on the MMIC, there are a MIM (Metal Insulator Metal) capacitor shown in FIG. 8A as the first example and an inter-digital capacitor shown in FIG. 8B as the second example.
As shown in FIG. 8A, the MIM capacitor has a structure in which two conductors 1, 2 are stacked on a substrate 4 via a dielectric layer 3 to oppose to each other, and has such a feature that a large capacitance can be obtained by a relatively small pattern area. Since normally the MIM capacitor employed in the MMIC is formed by using a thin film process, a dielectric layer (e.g., SiO2) formed by the chemical vapor deposition is employed as the dielectric layer 3, or the dielectric layer 3 can be formed with resin by coating a polyimide resin paste on a conductor being formed on the substrate, or a ceramic dielectric layer can be formed by coating the dielectric paste on the conductor formed on the substrate by using the sol-gel method, etc. and then firing it. Since the dielectric layer connected by the above method can be formed to have a thickness of about several xcexcm, it is easy to implement the capacitor having a large capacitance with a small area.
As shown in FIG. 8B, the inter-digital capacitor as the second example in the prior art has a structure in which comb-type electrodes 5, 6 are opposed to each other on the same surface of the substrate 4. In other words, the comb-type electrodes 5, 6 have a plurality of element electrodes 7, 8 respectively, and the plurality of element electrodes 7, 8 are opposed to each other on a surface of the substrate 4 along the surface direction to form a capacitance. Normally, the inter-digital capacitor employed in the MMIC is formed by forming a conductive film on an overall surface of the substrate 4 by virtue of the sputtering, etc., then coating photoresist on the conductive film, then exposing and developing a pattern to be formed onto the photoresist, and then etching a conductive film portion to be removed. Hence, since both the comb-type electrodes 5, 6 of the inter-digital capacitor can be formed by the same step, they have a structure which has small variation of a capacitance value being accomplished in mass production especially.
According to the MIM capacitor shown in FIG. 8A as the first example in the prior art, the capacitance value is varied according to a film thickness of the dielectric layer 3 formed between the capacitor electrodes 1, 2. For example, in the case of the MIM capacitor which is formed to have the dielectric layer 3 of 5 xcexcm thickness, the capacitance value to be formed is subjected to the variation of xc2x110% even if a film thickness of the dielectric layer 3 can be formed with a precision of xc2x10.5 xcexcm. Though a precision of the capacitor depends on a precision of the filter circuit, etc., normally such precision of the capacitor employed in the filter circuit, etc. must be restrained in the range of about xc2x15% of the target value of the capacitance value, and the higher precision of the capacitor is also requested in some cases. Accordingly, in order to achieve such precision, a precision of the film thickness in forming the dielectric layer 3 must be suppressed less than xc2x10.25 xcexcm. However, in order to form the dielectric layer 3 within the foregoing precision in mass production, there are problems that the film thickness is readily varied if any above-mentioned methods are used to form the dielectric layer 3 and that especially the capacitance value to be formed is ready to vary as the dielectric layer 3 is made thinner.
According to the inter-digital capacitor shown in FIG. 8B as the second example in the prior art, the stable capacitance value can be derived in mass production as mentioned above, but it is hard to form the large capacitance value. Therefore, in order to get the large capacitance, the patterns of the comb-type electrodes 5, 6 must be increased in size, and thus they are unsuitable for the narrow pattern regions. In addition, as the method of increasing the capacitance value, the clearances between the element electrodes 7, 8 of the comb-type electrodes 5, 6 which are opposed on the surface of the substrate 4 must be designed small.
However, clearances between the element electrodes 7, 8 are formed by the etching, as described above. Therefore, the etching conditions are strictly restricted if such clearances are set extremely narrow, so that there is a problem such that variation in forming the electrodes is caused. More particularly, due to slight variation of the etching conditions in mass production, the conductive film cannot be sufficiently etched and thus short-circuit between the element electrodes 7, 8 is caused. Conversely, due to overetching, the element electrodes 7, 8 are formed too narrow and thus the element electrodes 7, 8 are eliminated in some areas.
In view of the above problem, it is an object of the present invention to provide a capacitor employed in the MMIC and having a structure which is capable of increasing a capacitance in areas occupied by capacitor patterns, and also reducing the variation of the capacitance value in mass production.
In order to achieve this object, there are provided a capacitor of the present invention constructed by forming a substantial comb-type lower electrode on a substrate, then forming a dielectric layer on the lower electrode, and then forming a substantial comb-type upper electrode on the dielectric layer, wherein respective element electrodes of one of the lower electrode and the upper electrode are arranged in blank areas between respective element electrodes of the other of the lower electrode and the upper electrode.
Further, a capacitor of the present invention is constructed by forming a lower electrode on a substrate, then forming a dielectric layer on the lower electrode, and then forming an upper electrode on the dielectric layer, wherein one electrode of the lower electrode and the upper electrode has frame-like element electrodes which construct a continuous ladder shape while other electrode of the lower electrode and the upper electrode is formed like a comb shape, and element electrodes of the other electrode formed like the comb shape are placed in blank areas between the frame-like element electrodes of one electrode.
Furthermore, a capacitor of the present invention is constructed by forming a lower electrode on a substrate, then forming a dielectric layer on the lower electrode, and then forming an upper electrode on the dielectric layer, wherein one electrode of the lower electrode and the upper electrode has a plurality of loop portions, and element electrodes of an other electrode of the lower electrode and the upper electrode are placed in blank areas of the loop portions of the one electrode.
Also, in a capacitor of the present invention, a relationship
W1xe2x89xa7W2+2xc2x7W3
is satisfied, where a width of a blank area between element electrodes of one electrode is set to W1, a width of an element electrode of the other electrode is set to W2, and a maximum assumed displacement of electrode patterns of the other electrode from a designed position is set to xc2x1W3.
Moreover, in a capacitor of the present invention, the substrate is formed of ceramic dielectric substance, and the substrate has an external connecting electrode to be connected to an electrode formed on a mother board.
Still further, in a capacitor of the present invention, the external connecting electrode is formed on a surface of the substrate on which the lower electrode and the upper electrode are formed.
Still further, in a capacitor of the present invention, a solder precoat or a solder bump is formed on the external connecting electrode.
Still further, in a capacitor of the present invention, the lower electrode and the upper electrode are formed by using a photo-lithography technology respectively.
Still further, in a capacitor of the present invention, the dielectric layer is formed of resin material.
Still further, in a capacitor of the present invention, the dielectric layer is formed by dispersing and mixing high dielectric constant material, which consists of more than one of ceramic powder, glass powder, and high dielectric constant resin powder, into the resin material.
Still further, in a capacitor of the present invention, the dielectric layer is formed by coating a dielectric paste and firing it.
According to the present invention, the capacitor is constructed such that the element electrodes of one (the upper electrode or the lower electrode) of the comb-type electrodes are arranged in blank areas between the element electrodes or in the element electrodes of the other (the lower electrode or the upper electrode) of the comb-type electrodes. Thus, in the event that displacement of the opposing comb-type electrodes is caused, one sides of the element electrodes of one of the comb-type electrodes come close to the corresponding element electrodes of the other of the comb-type electrodes to then increase the capacitance value, but other sides of the element electrodes of one of the comb-type electrodes go away from the corresponding element electrodes of the other of the comb-type electrodes to then decrease the capacitance value. Therefore, there can be provided the capacitor which is capable of canceling variation of the capacitance value on both sides of the element electrodes, reducing variation of the capacitance value due to pattern displacement, and reducing variation of the capacitance value small.
In addition, since the dielectric layer is interposed between the underlying electrode and the overlying electrode, there is no possibility that short-circuit, etc. are not caused even if both electrodes are set in the close vicinity. Therefore, since both electrodes can be placed very closely, the capacitor having the large capacitance value can be fabricated.